Eilenach... could be seen far to the West...; but it was not suitable for a large beacon-fire, there being little space on its sharp summit. Hence the name Nardol "Fire-hilltop" of the next beacon westward; it was on the end of a high ridge, originally part of the Drúadan Forest, but long deprived of trees by masons and quarriers who came up the Stonewain Valley. Nardol was manned by a guard, who also protected the quarries; it was well-stored with fuel and at need a great blaze could be lit, visible on a clear night even as far as the last beacon (Halifirien) some hundred and twenty miles to the westward.

Unfinished Tales, Part 3, Ch 2, Cirion and Eorl and the Friendship of Gondor and Rohan: Notes, Note 51

And so King Théodendeparted from his own realm, and mile by mile the long road wound away, and the beacon hills marched past: Calenhad, Min-Rimmon, Erelas, Nardol. But their fires were quenched.

The Return of the King, LoTR Book 5, Ch 3, The Muster of Rohan

[It] had taken time for the Riders... to find paths... down into the hidden Stonewain Valley. It was late in the afternoon when the leaders came to wide grey thickets stretching beyond the eastward side of Amon Dîn, and masking a great gap in the line of hills that from Nardol to Dîn ran east and west. Through the gap the forgotten wain-road long ago had run down, back into the main horse-way from the City through Anórien....